Selenium rectifiers



Patented Mar. 16,1948

SELENIUM RECTIFIERS Wayne E. Blackburn, Wilkinsburg, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application November 10, 1943, Serial N- 509,818

1 5 Claims.

My invention relates to processes of making selenium rectifiers and, in particular relates to methods of reducing the leakage current which fiows through such rectifiers in their high resistance, i. e., normally non-conducting, direc' tion.

One object of my invention is to provide a method of reducing the losses due to currentheating in selenium rectifiers.

Another object of my invention is to provide a method of reducing current flow in the high resistance direction through selenium rectifiers,

Still another object of my invention is to provide a method of increasing the resistance of selenium rectifiers to current how in their high resistance direction without unduly increasing their resistance to current flow in their low re sistance direction.

Other objects of my invention will become apparent upon reading the following description. In accordance with the preferred form of my invention, I produce'selenium rectifiers by sandblasting a steel plate and thereafter nickel plating the surface. I- then dip the steel plate in molten selenium and remove the selenium in excess of a thin uniform film. One convenient way of carrying out this operation is to mount the steel plates on a spindle which is dipped into the molten selenium and withdrawn therefrom While rotating, thereby causing centrifugal force to remove the excessive amounts of selenium from the surface. I then deposit a contact on the free surface ofthe selenium, preferably by Schoop-spraying it with cadmium.

While I have described the above process as using selenium, it may be desirable to use selenmm in which has been dissolved about .1% of bromine. The above-described steps of producing selenium rectifiers' are described in my copending application Serial No. 509,817, filed November 10, 1943, for Method of making selenium rectifiers. I then anneal the unit thus formed at a temperature of between L70 and 185 0., preferably for a period of at least 6 hours. The foregoing method of producing selenium rectifiers is that described in my copending application Serial No. 509,817, filed November 10, 1943, for Method of making selenium rectifiers but the following procedure is likewise applicable to rectifiers in which the annealing process is carried out before the deposition of the contact layer in- 2 flow through the rectifier in the high resistance, or normally non-conductive direction, by impressing between the two terminal plates a direct-current voltage having a polarity which tends to cause current flow in the high resistance direction. In order to obtain best results. it is necessary that at some time during the course of the treatment the voltage should rise to a value greater than that to which the rectifier will have to withstand in its high resistance direction during service on alternating-current supplies. The application of such a forming voltage gradually increases the resistance to leakage current and does so without. causing a deleterious increase of resistance to the flow of current through the rectifier in the conductive direction.

However, Ihave iound that if a voltage of the .magnitude just described is to be applied. to the rectifier, it must be d'one so in accordance with certain rules of procedure; otherwise the wattage dissipated. during the forming process is likely to melt the contact electrode. It might appear that by. sufficiently cooling the contact electrode this difliculty could be overcome, but

I have found that if such cooling is resorted to, the forming operation has very little effect on the leakage current.

I have found, however, that if the temperaally raising this voltage to a value higher than said maximum value.

One very convenient way of carrying out the above-described procedure is toapply the lower value of forming voltage Just described to a rectifier which is provided with means for artificially cooling its contact layer. The amount of cooling effect is then adjusted until, with this lower. value of inverse voltage applied, the rectifier comes to equilibrium at the temperature of about C. It will be found that under the action of this voltage, the reslstance of the rectifier gradually in creases. In consequence of this increase, the heating effect of current flow grows smaller and it is necessary to raise the impressed'voltage if the unit is to be maintained at a temperature of 155 C. Such an increaseof voltage is accordingly made andcontinued preferably at such a actual service. To take a specific example, I have found that with rectifiers made in accordance with the foregoing description, the initial voltage across a single selenium layer should be about 12 volts and the application of this voltage should be continued with increasing values until it has reached 32 volts or more.

While I have described a specific embodiment of my invention in accordance with the patent statutes, the principles thereof are of broader application in ways that will be self-evident to those skilled in the art.

I claim as my invention:

1. The process of making selenium units which comprises depositing a layer of selenium on a metallic plate, forming a-contact layer on the free surface of said selenium, annealing the unit thus formed and thereafter applying a directcurrent voltage across the selenium in a direction tending to send current in' the high resistance direction while maintaining the temperature of said unit at about 155 C., said voltage having an initial value below the maximum which said unit must stand in the reverse current direction in service, and being gradually raised to a value higher than said maximum.

2. The process of malging selenium units which comprises depositing a layer of selenium on a metallic plate, forming a contact layer on the free surface of said selenium, annealing the unit thus formed and thereafter applying a directcurrent voltage across the selenium in a direction tending to send current in the high resistance direction while maintaining said unit at about the temperature at which its temperature-resistance coefficient in respect to current flow in the high resistance direction becomes positive, said voltage having an initial value below the maximum which said unit must stand in the reverse current direction in service, and being gradually raised'to a value higher than said maximum.

3. The method of forming selenium rectifiers which comprises sandblasting a steel plate, nickel plating it, dipping it while rotating into molten selenium and withdrawing it therefrom, providing a contact layer on the free surface of said selenium, annealing said unit and thereafter impressing across the selenium in its high resistance direction a direct-current voltage which is maintained at a value such that the temperature of the unit remains at about C.

4. The method of electrically forming a selenium unit comprising a layer or annealed selenium sandwiched between two conducting surfaces which comprises impressing across said layer in the high resistance direction a directcurrent voltage which is maintained at a value at least sufficient to insure a positive temperature-resistance coeflicient in respect to current flow in the high resistance direction through said selenium.

5. The method of electrically forming a selenium unit comprising a layer of annealed selenium sandwiched between two conducting surfaces which comprises impressing across said layer in the high resistance direction a directcurrent voltage which is maintained at a value which keeps the temperature of said unit at about 155 C.

WAYNE E. BLACKBURN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,229,807 Hoppe et al. Jan. 28, 1941 2,246,161 Ademstedt et al. June 17, 1941 2,261,725 Hoppe Nov. 4, 1941 2,279,187 Thompson Apr. 7, 1942 2,339,613 Becker et al. Jan. 18, 1944 OTHER REFERENCES Notes on Resistances of Selenium Cells Deposed in Certain Cases; an article from the Journal of the Optical Society of America, vol. 25, No. 1, pages 1, 2 and 3, Jan. 1935. 

